1. Field of the Invention
This invention relates to a silicone elastomer composition which is extrudable and which cures to a high strength, high durometer silicone elastomer.
2. Description of the Prior Art
Silicone elastomers can be made from many different polyorganosiloxanes, cure mechanisms and fillers. These silicone elastomers have been made from stiff gum polymers, as well as, water thin fluids. The curing mechanisms have ranged from organic peroxides to moisture sensitive means to radiation. A variety of fillers have also been used, such as reinforcing silica fillers to extending fillers. One unique silicone elastomeric composition can be prepared from a vinyl-containing polydiorganosiloxane, an organohydrogensilicon crosslinker, a platinum catalyst and optionally fillers. These compositions are desirable for many reasons, such as curable without leaving groups, can be cured either at room temperature or at elevated temperature, can be stabilized at room temperature for storage by a platinum catalyst inhibitor and can be made from either high viscosity or low viscosity polymers. Although these compositions are very useful and many commercial products are available, there are still some combinations of properties yet to be discovered. The present invention provides one such composition which has a useful combination of properties both in the uncured state and in the cured state. It is desirable to have readily extrudable compositions which can be extruded and molded at low pressure and be cured to elastomers with high strength and high durometer in relatively short periods of time to be practical commercially.
An organic peroxide, heat cured, organosiloxne composition is disclosed by Polmanteer in U.S. Pat. No. 3,122,516, which has improved hot strength. The hot strength, according to Polmanteer, is a synergistic effect obtained when reinforcing silica contains a specific range of vinyl substituted silyl units attached through siloxane linkages to a specific portion of the silicon atoms in the surface of the silica substrate and these vinyl substituted silyl units are present within the specified proportions. The modified silica filler defined by Polmanteer is described as containing 50 to 100 mol percent SiO.sub.2 units (W) and 0 to 50 mol percent RSiO.sub.1.5 units (X) which are particles having EQU R.sub.n SiO.sub.4-n/2
units (Y) and EQU R.sub.x (CH.sub.2 .dbd.CH)SiO.sub.3-x/2
units (Z) where there are 4 to 29.9 (Y) units and 0.1 to 2 (Z) units per 100 (W) and (X) units and a total of 6 to 30 (Y) and (Z) units per 100 (W) and (X) units. The rubber stocks described by Polmanteer permit vinyl in the organopolysiloxane and suggests that the best results are obtained using polymers having 99 to 99.95 inclusive mol percent dimethylsiloxane units and 0.05 to 1 inclusive mol percent methylvinylsiloxane units. Polmanteer teaches that from 20 to 100 parts of filler per 100 parts of polysiloxane gum must be used to achieve the improved hot strength, where the viscosity of the polymer and the desired amount of filler are in an inverse ratio to each other. However, Polmanteer does not suggest a composition of this invention which is a readily extrudable composition that cures to a high strength, high durometer elastomer wherein the composition requires a unique combination of a specific vinyl-siloxane polymer, a specific vinylsiloxane treated silica, an organohydrogensiloxane crosslinker and a platinum catalyst.
In U.S. Pat. No. 3,671,480, Wada et al. describe a heat curable elastomeric silicone composition comprising a mixture of two polydiorganosiloxanes, each of which contains vinyl unsaturation, a silica filler, a polyorganohydrogensiloxane and a platinum compound. The compositions described by Wada et al. are stated to cure to high tear strength, flame retardant, heat resistant elastomers with superior compression set. These compositions are not readily extrudable but require pressing into shape under substantial pressure and then cured.
Polmanteer et al. in U.S. Pat. No. 3,697,473 describe a composition which is fluid and consists essentially of a polydiorganosiloxane having two vinyl radicals per molecule and being terminated with triorganosiloxy group, a mixture of silicon compounds containing silicon-bonded hydrogen atoms, one compound containing two silicon-bonded hydrogen atoms per molecule and another compound containing 3 to 10 silicon-bonded hydrogen atoms per molecule and cured with a platinum catalyst, optionally a reinforcing silica. Although these compositions described by Polmanteer et al. are fluid, the cured elastomers have low modulus and a low durometer with high tensile strengths and elongations. Such compositions may be useful for many applications, but do not obtain the high durometers along with the high tear strengths and tensile strengths which are required in other applications.
In U.S. Pat. No. 3,884,866, Jeram et al. describe an organopolysiloxane composition which is stated as suited for low pressure injection molding and which is stated as having high tensile strength,, good elongation and high tear. These compositions described by Jeram et al. comprise 20 to 90 parts of a vinyl-containing high viscosity organopolysiloxane having a viscosity of 5,000 to 1,000,000 centipoise, 5 to 40 parts of a low viscosity vinyl-containing organopolysiloxane having a viscosity of 50 to 5,000 centipoise, 5 to 70 parts of filler and 0.1 to 50 parts per million of a platinum catalyst. This composition is cured by mixing 100 parts of it with 1 to 100 parts of a hydrogen silicone composition. These Jeram et al. compositions appear to suffer the same problems encountered with other prior art compositions. Jeram et al. teach that using small amounts of a treated silica filler, low durometer elastomers are obtained and increasing the amount of silica filler increases the durometer as well as the viscosity of the composition.
As can be seen by these prior art teachings, the adjustment of a composition to obtain one desired property results in the loss of another property. The result is that the desired property is achieved with a compromise of the other properties and sometimes all the properties are compromised.